Diabetes is known to cause high levels of glucose to circulate in the bloodstream due to a lack of, or resistance to, the hormone known as insulin. Several studies have shown that closely monitoring blood glucose levels can significantly decrease the long-term health effects of diabetes. Though conventional methods of personalized glucose monitoring are done by extracting blood from the fingertip, more patient-friendly methods of glucose monitoring are desired. One such method involves the use of optical polarimetry.
A schematic illustration where a Faraday cell used in a device for sensing glucose in a birefringent medium is shown in PRIOR ART FIG. 1 (which is a schematic illustration of one embodiment shown in the Cameron et al. U.S. Pat. No. 7,245,952 issued Jul. 17, 2007). While such devices can provide ultra-sensitivity and control with the ability to discern and control sub-millidegree rotations in the electric field of light, such devices have several drawbacks. For instance, the Faraday cells are comprised of bulky custom-wound coils thereon. In use, the overdriving can cause coil damage, and the coils are difficult to replace. Additionally, such devices are difficult to tailor-design for custom specifications. Moreover, modulator and compensator operation is provided by two separate devices that each require an expensive optical crystal that has a desirable Verdet constant. In general, a lower Verdet constant require a larger magnetic field. A material with a high Verdet constant is one that has a relatively large amount of polarization rotation under the application of a given magnetic field.
In the past, Faraday-based optical modulation and compensation were performed separately. These Faraday modulators and these Faraday compensators each incorporated their own optical crystal—necessary in order to produce the needed axial magnetic field component for a given rotational depth by winding custom-fabricated inductive coils and placing each crystal in the center of its own coil. Having a separate Faraday modulator and a Faraday compensator requires multiple optical crystals. Proper alignment of these separate optical crystals is problematic and challenging since any light beam diverges after going through the first crystal. The use of multiple optical crystals also requires matching (e.g., optically compatible) optical crystals in order to achieve optical performance. Thus, even crystals purchased from the same vendor often have slight variations unless they came from the same fabrication batch. These variations, in turn, cause a stress birefringence in the light beam passing through the crystals. Furthermore, it is time-consuming and expensive to obtain sufficiently matched optical crystals.
One previous attempt, described in Gobeli U.S. Pat. No. 6,246,893, to combine both Faraday modulation and Faraday compensation with a single crystal mixed the two electric signals, and then applied the combined electric signal to a single inductive coil. One problem with this approach is that mixing the different types of electrical signals (i.e., AC & DC) can be problematic on the high gain AC amplifier side, as these amplifiers often have an inherent DC offset, or drift, which tends to confound or mask the considerably smaller DC compensation drive term needed to achieve optimal performance. As a result, such devices utilizing such an approach cannot be implemented for accurate sensing of certain analytes having a small optical rotation, such as glucose.
To date, there are still many obstacles to the development of a commercially viable and manufacturable product suitable for optical polarimetric glucose detection. Thus, there is a need in the art for improved and more cost-efficient systems for polarimetric and other optical sensing applications.